Optical device with low-loss thermally tunable closed-curve optical waveguide

ABSTRACT

Disclosed is a photonic structure and associated method. The structure includes a closed-curve waveguide having a first height, as measured from the top surface of an insulator layer, and an outer curved sidewall that extends essentially vertically the full first height (e.g., to minimize signal loss). The structure includes a closed-curve thermal coupler and a heating element. The closed-curve thermal coupler is thermally coupled to and laterally surrounded by the closed-curve waveguide and has a second height that is less than the first height. In some embodiments, the closed-curve waveguide and the closed-curve thermal coupler are continuous portions of the same semiconductor layer having different thicknesses. The heating element is thermally coupled to the closed-curve thermal coupler and thereby indirectly thermally coupled to the closed-curve waveguide. Thus, the heating element is usable for thermally tuning the closed-curve waveguide via the closed-curve thermal coupler to minimize any temperature-dependent resonance shift (TDRS).

BACKGROUND Field of the Invention

The present invention relates to temperature-sensitive closed-curveoptical waveguides and, more particularly, to embodiments of a photonicstructure including an optical device with a low-loss, thermallytunable, closed-curve optical waveguide.

Description of Related Art

An optical ring resonator includes multiple optical waveguides. In thesimplest of optical ring resonators, the optical waveguides include abus waveguide (i.e., an optical waveguide with discrete ends includingan input end and an output end) and a closed-curve waveguide (i.e., anoptical waveguide with a complete loop or ring shape having no discreteends), which is spatially separated from but optically coupled to thebus waveguide. Light signals can enter the bus waveguide at the inputend. Due to optical coupling, some light signals will pass into theclosed-curve waveguide from the bus waveguide and some light signalswill pass from the closed-curve waveguide into the bus waveguide. Lightsignals will also exit the bus waveguide at the output end. However,within the closed-curve waveguide, light signals of a specific resonantwavelength for that closed-curve waveguide will make repeated roundtripsthrough the closed-curve waveguide, building up intensity, due, forexample, to constructive interference. As a result, the light signalsthat pass from the closed-curve waveguide into the bus waveguide will,predominantly, have the specific resonant wavelength. Thus, such a ringresonator can be effectively employed as a filter. However, dependingupon the properties of the core material used, closed-curve waveguidescan be thermally sensitive. That is, they can exhibittemperature-dependent resonance shifts (TDRS). For example, closed-curvesilicon waveguides are known to be thermally sensitive and the potentialTDRS can be, for example, approximately 70 picometers per Kelvin (pm/K)or more. To minimize TDRS, many photonic structures that includeclosed-curve waveguide(s) also include corresponding heater(s) tothermally tune (i.e., heat) the closed-curve optical waveguide(s).Unfortunately, currently available configurations for such photonicstructures tend to exhibit performance degradation (e.g., optical powerloss).

SUMMARY

Disclosed herein are embodiments of a photonic structure. The photonicstructure can include an optical device. The optical device can includeone or more optical waveguides including at least one closed-curvewaveguide. For example, in some embodiments, the optical device can be aring resonator, which includes at least one bus waveguide and at leastone closed-curve waveguide positioned laterally adjacent to the buswaveguide and, more particularly, spatially separated from but opticallycoupled to the bus waveguide. The closed-curve waveguide can have afirst height and, particularly, having an outer curved sidewall thatextends vertically the first full height. The photonic structure canfurther include a closed-curve thermal coupler. This closed-curvethermal coupler can be laterally surrounded by and thermally coupled tothe closed-curve waveguide. The closed-curve thermal coupler can furtherhave a second height that is less than the first height of theclosed-curve waveguide. In some embodiments, the closed-curve waveguideand the closed-curve thermal coupler are continuous portions of the samesemiconductor layer (e.g., the same silicon layer) having differentthicknesses. The photonic structure can further include a heatingelement. The heating element can be adjacent to the closed-curve thermalcoupler and, more particularly, thermally coupled to the closed-curethermal coupler and thereby indirectly thermally coupled to theclosed-curve waveguide. As a result, heat energy generated and output bythe heating element can pass into the closed-curve thermal coupler, canpass through the closed-curve thermal coupler, and can further pass intothe closed-curve waveguide. Thus, the heating element can be used tothermally tune the closed-curve waveguide via the closed-curve thermalcoupler in order to minimize any temperature-dependent resonance shift(TDRS).

Also disclosed herein are embodiments of a method of forming theabove-described photonic structure. The method can include forming anoptical device. The process of forming the optical device can includeforming one or more optical waveguides including at least oneclosed-curve waveguide. For example, in some embodiments, the process offorming the optical device can include forming a ring resonator, whichincludes at least one bus waveguide and at least one closed-curvewaveguide positioned laterally adjacent to the bus waveguide and, moreparticularly, spatially separated from but optically coupled to the buswaveguide. The closed-curve waveguide of the optical device can beformed so as to have a first height and, particularly, so as to have anouter curved sidewall that extends essentially vertically the full firstheight. The method can further include forming a closed-curve thermalcoupler. This closed-curve thermal coupler can be formed so as to belaterally surrounded by and thermally coupled to the closed-curvewaveguide and further so as to have a second height that is less thanthe first height of the closed-curve waveguide. In some embodiments, theclosed-curve waveguide and the closed-curve thermal coupler are formedas continuous portions of the same semiconductor layer (e.g., the samesilicon layer) having different thicknesses. The method can furtherinclude forming a heating element. The heating element can be formedadjacent to the closed-curve thermal coupler such that it is thermallycoupled to the closed-curve thermal coupler and thereby indirectlythermally coupled to the closed-curve waveguide. The method can furtherinclude using the heating element to thermally tune the closed-curvewaveguide via the closed-curve thermal coupler (e.g., using the heatingelement to generate and output heat energy, which passes into andthrough the closed-curve thermal coupler and which further passes intothe closed-curve waveguide) in order to minimize anytemperature-dependent resonance shift (TDRS).

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The present invention will be better understood from the followingdetailed description with reference to the drawings, which are notnecessarily drawn to scale and in which:

FIG. 1 is a layout diagram of a photonic structure;

FIGS. 1A-1D are cross-section diagrams illustrating alternativeembodiments of the photonic structure shown in FIG. 1 ;

FIG. 2 is a layout diagram of another photonic structure;

FIGS. 2A-2D are cross-section diagrams illustrating alternativeembodiments of the photonic structure shown in FIG. 2 ;

FIG. 3 is a layout diagram of yet another photonic structure;

FIGS. 3A-3B are cross-section diagrams illustrating alternativeembodiments of the photonic structure shown FIG. 3 ;

FIG. 4 is a flow diagram illustrating method embodiments for forming aphotonic structure;

FIGS. 5A-5C are cross-section diagrams illustrating exemplary processsteps for forming the photonic structure embodiment shown in FIG. 1A;

FIGS. 6A-6C are cross-section diagrams illustrating exemplary processsteps for forming the photonic structure embodiment shown in FIG. 1B;

FIGS. 7A-7C are cross-section diagrams illustrating exemplary processsteps for forming the photonic structure embodiment shown in FIG. 1C;and

FIGS. 8A-8C are cross-section diagrams illustrating exemplary processsteps for forming the photonic structure embodiment shown in FIG. 1D.

DETAILED DESCRIPTION

As mentioned above, closed-curve waveguides (i.e., optical waveguideswith a complete loop or ring shape having no discrete ends) can bethermally sensitive depending upon the properties of the core materialsused. That is, they can exhibit temperature-dependent resonance shifts(TDRS). For example, closed-curve silicon waveguides are known to bethermally sensitive and the potential TDRS can be, for example,approximately 70 picometers per Kelvin (pm/K) or more. To minimize TDRS,many photonic structures that include closed-curve optical waveguide(s)also include corresponding heater(s) to thermally tune (i.e., heat) theclosed-curve optical waveguide(s). Unfortunately, currently availableconfigurations for such photonic structures tend to exhibit performancedegradation (e.g., optical power loss).

In view of the foregoing, disclosed herein are embodiments of a photonicstructure that includes an optical device with a low-loss, thermallytunable, closed-curve optical waveguide. More particularly, disclosedherein are embodiments of a photonic structure. This photonic structurecan include an optical device with one or more optical waveguidesincluding at least one closed-curve waveguide. The closed-curvewaveguide can have a first height, as measured from the top surface ofan insulator layer, and an outer curved sidewall that extendsessentially vertically the full first height (e.g., so as to minimizesignal loss and improve mode confinement, as discussed further below).The photonic structure can also include a closed-curve thermal coupler,which is thermally coupled to and laterally surrounded by theclosed-curve waveguide and which has a second height that is less thanthe first height. In some embodiments, the closed-curve waveguide andthe closed-curve thermal coupler can be continuous portions of the samesemiconductor layer (e.g., the same silicon layer or, alternatively, thesame polysilicon, germanium, or silicon germanium layer) havingdifferent thicknesses. Finally, the photonic structure can include aheating element, which is thermally coupled to the closed-curve thermalcoupler (e.g., in the same design level or in a different design level)and thereby indirectly thermally coupled to the closed-curve waveguidevia the closed-curve thermal coupler. In such a photonic structure, theheating element can be used to thermally tune the closed-curve waveguidevia the closed-curve thermal coupler in order to minimize anytemperature-dependent resonance shift (TDRS). Also disclosed herein aremethod embodiments for forming the above-described photonic structure.

More particularly, disclosed herein are various embodiments of aphotonic structure. For example, see the photonic structure embodiments100A-100D shown in the layout diagram of FIG. 1 and further illustratedin the alternative cross-section diagrams of FIGS. 1A-1D, respectively;see the photonic structure embodiments 200A-200D shown in the layoutdiagram of FIG. 2 and further illustrated the alternative cross-sectiondiagrams of FIGS. 2A-2D, respectively; and see the photonic structureembodiments 300A-300B shown in the layout diagram of FIG. 3 and furtherillustrated in the alternative cross-section diagrams of FIGS. 3A-3B.

Each of the photonic structure embodiments 100A-100D, 200A-200D,300A-300B can include a substrate 101, 201, 301. The substrate 101, 201,301 can be, for example, a semiconductor substrate, such as a siliconsubstrate. Each of the photonic structure embodiments 100A-100D,200A-200D, 300A-300B can further include an insulator layer 102, 202,302 on the substrate 101, 201, 301 and an optical device 199, 299, 399,which includes one or more optical waveguides and, more particularly,which includes at least one closed-curve waveguide 120, 220, 320, on theinsulator layer 102, 202, 302.

Each optical waveguide of the optical device 199, 299, 399 can be, forexample, a semiconductor waveguide, such as a silicon waveguide or,alternatively, a polysilicon waveguide, a germanium waveguide, a silicongermanium waveguide, or any other type of waveguide having a corematerial with a temperature-dependent refractive index.

Those skilled in the art will recognize that, to facilitate opticalsignal propagation through an optical waveguide, the waveguide material(also referred to as the core material) should have a first refractiveindex and should be surrounded by cladding material that has a secondrefractive index that is less than the first refractive index. Forexample, silicon has a refractive index that is a function of bothtemperature and wavelength. That is, given a wavelength of 1 micron, therefractive index of silicon can range from approximately 3.52 toapproximately 3.57 at temperatures ranging from approximately 50K toapproximately 295K, respectively. However, given a wavelength of 5micron, the refractive index of silicon is lower and can range fromapproximately 3.39 to approximately 3.43 at temperatures ranging fromapproximately 50K to approximately 295K, respectively. Thus, if theoptical waveguide(s) of the optical device 199, 299, 399 are silicon,then the insulator layer 102, 202, 302, which is immediately adjacent tothe bottom surfaces of the optical waveguide(s) and which will functionas cladding material, could be, for example, a silicon dioxide layerwith a refractive index of less than 1.6, a silicon nitride layer with arefractive index of less than 2.1, or a layer of any other suitableinsulator material with a refractive index that is less than the lowestrefractive index of silicon.

As mentioned above, the optical device 199, 299, 399 can include one ormore optical waveguides with at least one optical waveguide being aclosed-curve waveguide. A “closed-curve” waveguide refers to a waveguidewith a complete loop or ring shape having no discrete ends such thatoptical signals can make repeated roundtrips through the waveguide. Forpurposes of illustration, the closed-curve waveguide 120, 220, 320 isshown in the figures as having an elliptical ring shape with no discreteends. However, it should be understood that the figures are not intendedto be limiting and that, alternatively, the closed-curve waveguide 120,220, 320 could have any other complete loop or ring shape (e.g., acircular ring shape, an oval ring shape, a racetrack ring shape, etc.)with no discrete ends. In any case, the closed-curve waveguide 120, 220,320 can have a bottom surface immediately adjacent to the top surface ofthe insulator layer 102, 202, 302 and a top surface that is opposite andessentially parallel to the bottom surface. Thus, the closed-curvewaveguide 120, 220, 320 can have a first height 125, 225, 325, asmeasured from the top surface of the insulator layer 102, 202, 302.

The closed-curve waveguide 120, 220, 320 can have an outer curvedsidewall 121, 221, 321, which defines the outer boundary of thewaveguide and which extends essentially vertically the full first height(i.e., from the top surface of the insulator layer 102, 202, 302 to thetop surface of the waveguide). The closed-curve waveguide 120, 220, 320can also include an inner curved sidewall 122, 222, 322, which isopposite the outer curved sidewall 121, 221, 321 and which defines theinner boundary of the waveguide. The dimensions of the closed-curvewaveguide 120, 220, 320 can be customized to achieve desired results.For example, the height and width of the closed-curved waveguide can becustomized to facilitate propagation of light signals having wavelengthswithin a given wavelength range. Furthermore, the circumference of theclosed-curve waveguide can be customized to achieve a specific resonantwavelength and to set how often resonance occurs. As mentioned above,the resonant wavelength of a closed-curve waveguide refers to thewavelength of light signals that will make repeated roundtrips throughthe closed-curve waveguide, building up intensity. Techniques forcustomizing the dimensions of waveguides are well known in the art and,thus, the details have been omitted from this specification in order toallow the reader to focus on the salient aspects of the disclosedembodiments. It should be noted that having an outer curved sidewallthat is essentially vertical from the top surface of the insulator layer102, 202, 302 to the top surface of the closed-curve waveguide minimizesloss of signals with the specific resonant wavelength through the outercurved sidewall 121, 221, 321. Low loss is extremely important fordevice performance when circumference is small resulting in tight bends.

In some embodiments, the optical device 199, 299, 399 can be a ringresonator. As discussed above, a ring resonator can include a buswaveguide 110, 210, 310 positioned laterally adjacent and opticallycoupled to a closed-curve waveguide 120, 220, 320, as described above. A“bus” waveguide refers to a waveguide with discrete ends including aninput end 111, 211, 311 and an output end 112, 212, 312. For purposes ofillustration, the bus waveguide 110, 210, 310 is shown in the figures asincluding an essentially linear path between the input end and theoutput end. However, it should be understood that the figures are notintended to be limiting and that, alternatively, the bus waveguide 110,210, 310 could include a non-linear path between the input end and theoutput end. That is, alternatively, the bus waveguide 110, 210, 310could include one or more curves. In any case, the bus waveguide 110,210, 310, like the closed-curve waveguide 120, 220, 320, can have abottom surface that is immediately adjacent to the top surface of theinsulator layer 102, 202, 302 and a top surface that is opposite andparallel to the bottom surface. The bus waveguide 110, 210, 310 can, forexample, have the same first height as the closed-curve waveguide 120,220, 320 and parallel sidewalls that extend essentially vertically thefull first height (i.e., from the top surface of the insulator layer102, 202, 302 to the top surface of the waveguide) (as shown).Alternatively, the bus waveguide 110, 210, 310 can have a differentheight than the closed-curve waveguide 120, 220, 320 and parallelsidewalls that extend essentially vertically to this different height.In any case, the bus waveguide 110, 210, 310 can be positioned laterallyadjacent to a section of the outer curved sidewall 121, 221, 321 of theclosed-curve waveguide 120, 220, 310 and, more particularly, can bespatially separated from but optically coupled to the closed-curvewaveguide 120, 220, 320 at one section of the outer curved sidewall 121,221, 321.

It should be noted that the section of the outer curved sidewall of theclosed-curve waveguide that is optically coupled to the bus waveguidecan be a curved section (as illustrated). Alternatively, the section ofthe outer curved sidewall of the closed-curve waveguide that isoptically coupled to the bus waveguide can be a linear section (e.g., inthe case of a racetrack shaped closed-curve waveguide) (not shown).

In such a ring resonator, light signals can enter the bus waveguide 110,210, 310 at the input end 111, 211, 311. Due to optical coupling withthe closed-curve waveguide 120, 220, 320, some light signals can passfrom the bus waveguide 110, 210, 310 into the closed-curve waveguide120, 220, 320 and some light signals can pass from the closed-curvewaveguide 120, 220, 320 into the bus waveguide 110, 210, 310. Morespecifically, the bus waveguide and the closed-curve waveguide can beplaced sufficiently close to create an evanescent field between the twowaveguides and this evanescent field allows some light signals to passfrom the bus waveguide to the closed-curve waveguide and vice versa. Thecreation of such an evanescent field between the adjacent waveguides andthereby the likelihood of the optical coupling of the adjacentwaveguides (i.e., coupling that facilitates the transmission of lightsignals between the adjacent waveguides) is dependent upon at least thedistance between the waveguides, the coupling length (i.e., the lengthof the portions of the adjacent waveguides separated by the distance),and the refractive index of the medium between the waveguides. Thecloser the distance, the more likely the optical coupling; the longercoupling length, the more likely the optical coupling; etc. In any case,light signals can also exit the bus waveguide 110, 210, 310 at theoutput end 112, 212, 312. However, within the closed-curve waveguide120, 220, 320, only light signals of a specific resonant wavelength forthat closed-curve waveguide will make repeated roundtrips through theclosed-curve waveguide, building up intensity, due to constructiveinterference. As a result, the light signals that pass from theclosed-curve waveguide 120, 220, 320 into the bus waveguide 110, 210,310 and out at the output end 112, 212, 312 of the bus waveguide 110,210, 310 will, predominantly, have the specific resonant wavelength.However, as discussed above, closed-curve waveguides are known to bethermally sensitive. That is, they are known to exhibittemperature-dependent resonance shifts (TDRS). The potential TDRS canbe, for example, approximately 70 picometers per Kelvin (pm/K) or more.

Therefore, to minimize temperature-dependent resonance shifts (TDRS) inthe closed-curve waveguide 120, 220, 320 (i.e., to avoidtemperature-dependent variations in the resonant wavelength of theclosed-curve waveguide 120, 220, 320) each photonic structure embodiment100A-100D, 200A-200D, 300A-300B can further include a heating element140, 240, 340 and a thermal coupler 130, 230, 330, which indirectlythermally couples the heating element 140, 240, 340 to the closed-curvewaveguide 120, 220 320 for thermal tuning.

More specifically, each photonic structure embodiment 100A-100D,200A-200D, 300A-300B can further include a closed-curve thermal coupler130, 230, 330 with a bottom surface immediately adjacent to the topsurface of the insulator layer 102, 202, 302 and a top surface oppositeand essentially parallel to the bottom surface. For purposes of thisdisclosure, a “thermal coupler” refers to a non-contacted passivecomponent made of any suitable non-metallic electrically insulativethermally conductive material through which heat energy can betransmitted without also transmitting electric current. Exemplarythermal coupler materials are discussed in greater detail below withrespect to the specific photonic structure embodiments.

The closed-curve thermal coupler 130, 230, 330 can be smaller than theclosed-curve waveguide with essentially the same complete loop or ringshape (e.g., a circular ring shape, an oval ring shape, an ellipticalring shape, a racetrack ring shape, etc.) and can further be laterallysurrounded by the closed-curve waveguide 120, 220, 320. Morespecifically, the closed-curve thermal coupler 130, 230, 330 can bepositioned laterally adjacent to the inner curved sidewall 122, 222, 322of the closed-curve waveguide 120, 220, 320 and can abut or at least besufficiently close to that inner curved sidewall 122, 222, 322 so thatit is thermally coupled to the closed-curve waveguide 120, 220, 320.More specifically, the closed-curve thermal coupler can be placedimmediately adjacent to or sufficiently close to the inner curvedsidewall of the closed-curve waveguide so that heat energy can pass fromthe closed-curve thermal coupler to the closed-curve waveguide, therebyraising the temperature of the closed-curve waveguide. The likelihood ofthe thermal coupling of the closed-curve thermal coupler to theclosed-curve waveguide depends upon at least the distance between theclosed-curve thermal coupler and closed-curve waveguide, the couplinglength (length of the portions of the adjacent waveguides separated bythe distance), and the thermal conductivity of any medium between theclosed-curve thermal conductor and the closed-curve waveguide. Thecloser the distance, the more likely and the better the thermalcoupling; longer coupling length, the more likely and the better thethermal coupling; etc. Furthermore, the closed-curve thermal coupler130, 230, 330 can have a second height 135, 235, 335 (e.g., as measuredfrom the top surface of the insulator layer 102, 202, 302 to the top ofthe thermal coupler) that is less than the first height 125, 225, 325 ofthe closed-curve waveguide 120, 220, 320. In exemplary embodiments, thesecond height 135, 235, 335 can be less than half the first height. Forexample, the first height 125, 225, 325 of the closed-curve opticalwaveguide 120, 220, 320 could be 220 nm and the second height 135, 235,335 of the adjacent closed-curve thermal coupler 130, 230, 330 can be100 nm. Thus, in each of the photonic structure embodiments, theclosed-curve thermal coupler 130, 230, 330 is positioned laterallyadjacent to the lower portion only of the inner curved sidewall 122,222, 322 of the closed-curve waveguide 120, 220, 320 such that the innercurved sidewall 122, 222, 322 extends vertically above the level of thetop surface of the closed-curve thermal coupler 130, 230, 330.

Each photonic structure embodiment 100A-100D, 200A-200D, 300A-300B canfurther include a heating element 140, 240, 340 and at least twocontacts 149, 249, 349 on the heating element 140, 240, 340. Forpurposes of this disclosure, a “heating element” refers to a resistormade of any suitable conductive material through which electric currentflows in response to a voltage differential at the contacts and isconverted into heat energy. Those skilled in the art will recognize thatthe direction and amount of current flow will depend upon the voltagedifferential. Furthermore, the amount of heat generated per unit lengthwill depend upon the material used and the current density (which is afunction of the cross-sectional area of the heating element). Exemplaryheating element materials are discussed in greater detail below withregard to the specific embodiments. In any case, the heating element140, 240, 340 can be spatially separated from the closed-curve waveguide120, 220, 320. The heating element 140, 240, 340 can further be adjacentto the closed-curve thermal coupler 130, 230, 330 and, moreparticularly, can abut or at least be sufficiently close to theclosed-curve thermal coupler 130, 230, 330 so that it, like theclosed-curve waveguide, is thermally coupled to the closed-curve thermalcoupler 130, 230, 330. That is, the heating element can be placedimmediately adjacent to or sufficiently close to the closed-curvethermal coupler so that heat energy can pass from the heating element toand through the closed-curve thermal coupler to raise the temperature ofthe closed-curve thermal coupler. The likelihood of the thermal couplingof the closed-curve thermal coupler to the closed-curve waveguidedepends upon at least the distance between the closed-curve thermalcoupler and closed-curve waveguide, the coupling length (length of theportions of the adjacent waveguides separated by the distance), and thethermal conductivity of any medium between the closed-curve thermalconductor and the closed-curve waveguide. The closer the distance, themore likely and the better the thermal coupling; longer coupling length,the more likely and the better the thermal coupling; etc. It should benoted that, due to similar thermal coupling between the closed-curvethermal coupler and the closed-curve waveguide, the heat energy canfurther pass from the closed-curve thermal coupler into the closed-curvethermal waveguide to raise the temperature of the closed-curvewaveguide. Thus, the heating element 140, 240, 340 is indirectlythermally coupled to closed-curve waveguide 120, 220, 320 via theclosed-curve thermal coupler 130, 230, 330 and can be used to thermallytune the closed-curve waveguide 120, 220, 320 via the closed-curvethermal coupler 130, 230, 330 in order to minimize anytemperature-dependent resonance shift (TDRS). In other words, heatenergy can be generated and output by the heating element 140, 240, 340and, due to thermal coupling, can be passed into the closed-curvethermal coupler 130, 230, 330, can travel through the closed-curvethermal coupler 130, 230, 330, and can pass from the closed-curvethermal coupler 130, 230, 330 into the lower portion of the closed-curvewaveguide 120, 220, 320. That is, heat energy can be transferred fromthe heating element through the closed-curve thermal coupler and intothe closed-curve thermal waveguide. The amount of heat energy can bepredetermined to ensure that the temperature of the closed-curvewaveguide 120, 220, 320 is maintained at a specific temperature orwithin a specific temperature range so as to ensure that light signalsof the specific resonant wavelength will build up intensity as they makerepeated roundtrips through the closed-curve waveguide 120, 220, 320.

The various photonic structure embodiments 100A-100D, 200A-200D,300A-300B disclosed herein vary with regard to the materials used forthe different components (e.g., for the waveguide(s), the thermalcoupler, and the heating element) and/or with regard to the relativeplacement of the different components (e.g., the waveguide(s), thethermal coupler, and the heating element).

For example, referring to FIG. 1 and FIGS. 1A-1D, in the photonicstructure embodiments 100A-100D, the heating element 140 can be at thesame design level as the closed-curve thermal coupler 130 and theclosed-curve waveguide 120. Furthermore, the closed-curve thermalcoupler 130 can be positioned laterally between and immediately adjacentto both the closed-curve waveguide 120 and the heating element 140 to bethermally coupled to both the closed-curve waveguide 120 and the heatingelement 140.

More specifically, in the photonic structure embodiments 100A-100D, theheating element 140 can have a bottom surface immediately adjacent tothe top surface of the insulator layer 102. The heating element 140 canbe smaller than the closed-curve thermal coupler 130 with essentiallythe same loop or ring shape (e.g., a circular ring shape, an oval ringshape, an elliptical ring shape, a racetrack ring shape, etc.) and canfurther be laterally surrounded by and immediately adjacent to an innercurved sidewall of the closed-curve thermal coupler 130 such that thesethree components (i.e., the closed-curve waveguide 120, the closed-curvethermal coupler 130 and the heating element 140) essentially formconcentric, abutting, ring shapes.

It should be noted that the heating element 140 can be a complete loopor ring shape with no discrete ends (as shown). Alternatively, theheating element 140 could have at least one small segment removed so asto have an open-loop or ring shape with discrete ends (not shown).Alternatively, heating element 140 could be segmented with the segmentsforming the loop or ring shape and with each segment having discretecontacted ends (e.g., so that different amounts of heat energy could beapplied to different portions of the closed-curve thermal coupler andthereby to different portions of the closed-curve waveguide) (notshown).

In the photonic structure embodiment 100A (see FIG. 1A), theclosed-curve waveguide 120, the closed-curve thermal coupler 130, andthe heating element 140 can include continuous portions 103.1-103.3 ofthe same semiconductor layer 103 on the insulator layer 102. That is,the closed-curve waveguide 120 can be a first portion 103.1 of thesemiconductor layer 103, which has a first thickness (i.e., see thefirst height 125). The closed-curve thermal coupler 130 can be a secondportion 103.2 of the semiconductor layer 103 (also referred to herein asa recessed portion or slab portion), which is continuous with the firstportion 103.1 but which has been recessed (i.e., etched back) so as tohave a second thickness (i.e., see the second height 135) that is lessthan the first portion (i.e., the second portion 103.2 is thinner thanthe first portion 103.2). The heating element 140 can include a thirdportion 103.3 of the semiconductor layer 103, which is continuous withthe second portion 103.2, and can further include a metal silicide layer145 on the third portion 103.3. The semiconductor layer 103 can be, forexample, a silicon layer. Alternatively, the semiconductor layer couldbe some other type of semiconductor layer suitable for optical waveguideformation and having a temperature-dependent refractive index including,but not limited to, a polysilicon layer, a germanium layer, or a silicongermanium layer. The metal silicide layer 145 can be, for example, acobalt silicide (CoSi) layer, a nickel silicide (NiSi) layer, a tungstensilicide (WSi) layer, a titanium silicide (TiSi) layer, or any othersuitable metal silicide layer. Optionally, the metal silicide layer 145could be doped with N-type or P-type dopants for reduced resistance.

In the photonic structure embodiment 100B (see FIG. 1B), theclosed-curve waveguide 120 and the closed-curve thermal coupler 130 canbe continuous portions 103.1-103.2 of the same semiconductor layer 103(e.g., the same silicon layer or, alternatively, the same polysilicon,germanium, or silicon germanium layer, as described above with respectto the photonic structure embodiment 100A). However, in this case, theheating element 140 can be some other metal or metal alloy resistiveelement 141 formed on the insulator layer 102 abutting the inner curvedsidewall of the closed-curve thermal coupler 130. This resistive element141 can be made, for example, of tungsten, aluminum, nickel, titanium,tantalum, cobalt, copper, or alloys thereof.

In the photonic structure embodiments 100C (see FIG. 1C) and 100D (seeFIG. 1D), the closed-curve waveguide 120 can include a discrete portion103.1 of a semiconductor layer 103 (e.g., a silicon layer or,alternatively, a polysilicon, germanium, or silicon germanium layer, asdescribed above with respect to the photonic structure embodiment 100A).The closed-curve thermal coupler 130 can be some other non-metallicelectrically insulative thermally conductive feature 104 formed on theinsulator layer 102 and positioned laterally between and abutting theclosed-curve waveguide 120 and the heating element 140. For example, theclosed-curve thermal coupler 130 could be a thin layer of polysilicon,silicon, nitride, boron nitride, silicon carbide, or diamond on theinsulator layer 102 and extending laterally between and abutting theclosed-curve waveguide 120 and the heating element 140. The heatingelement 140 can include another discrete portion 103.3 of thesemiconductor layer 103 positioned laterally immediately adjacent to theclosed-curve thermal coupler 130 and can further include a metalsilicide layer 145 on the discrete patterned portion 103.3 of thesemiconductor layer, as described above with respect to the photonicstructure embodiment 100A (e.g., see the photonic structure embodiment100C of FIG. 1C). Alternatively, the heating element 140 can be someother metal or metal alloy resistive element 141 formed on the insulatorlayer 102 abutting the closed-curve thermal coupler 130, as describedabove with respect to the photonic structure embodiment 100B (e.g., seethe photonic structure embodiment 100D of FIG. 1D).

In each of the photonic structure embodiments 100A-100D, the opticaldevice 199 can be a ring resonator. The ring resonator can include atleast one closed-curve waveguide 120 and at least one bus waveguide 110adjacent to the closed-curve waveguide 120. In this case, the buswaveguide can be a discrete portion 103.4 of the semiconductor layer103, which is adjacent to but spatially separated from the closed-curvewaveguide 120 (i.e., from the first portion 103.1 of the semiconductorlayer 103) and optically coupled thereto.

Referring to FIG. 2 and FIGS. 2A-2D, in the photonic structureembodiments 200A-200D, the heating element 240 can be at the same designlevel as the closed-curve thermal coupler 230 and the closed-curvewaveguide 220. Furthermore, the closed-curve thermal coupler 230 can bepositioned laterally between the closed-curve waveguide 220 and theheating element 240. However, in this case, the closed-curve thermalcoupler 230 can be physically separated from the closed-curve waveguide220 and/or from the heating element 240 by space(s) 250. It should beunderstood that the distances between the closed-curve thermal coupler230 and the closed-curve waveguide 220 and the heating element 240(i.e., the widths of any space(s) 250) should be sufficiently small toensure so that the closed-curve thermal coupler 230 is still thermallycoupled to both closed-curve waveguide 220 on one side and the heatingelement 240 on the other.

More particularly, in the photonic structure embodiments 200A-200D, theheating element 240 can have a bottom surface immediately adjacent tothe top surface of the insulator layer 202. The heating element 240 canbe smaller than the closed-curve thermal coupler 230 with essentiallythe same loop or ring shape (e.g., a circular ring shape, an oval ringshape, an elliptical ring shape, a racetrack ring shape, etc.) and canfurther be laterally surrounded by the closed-curve thermal coupler 230such that these three components (i.e., the closed-curve waveguide 220,the closed-curve thermal coupler 230 and the heating element 240)essentially form concentric ring shapes with at least two of these threering shapes being separated by a space 250.

It should be noted that the heating element 240 can be a complete loopor ring shape with no discrete ends (as shown). Alternatively, theheating element 240 could have at least one small segment removed so asto have an open-loop or ring shape with discrete ends (not shown).Alternatively, heating element 240 could be segmented with the segmentsforming the loop or ring shape and with each segment having discretecontacted ends (e.g., so that different amounts of heat energy could beapplied to different portions of the closed-curve thermal coupler andthereby to different portions of the closed-curve waveguide) (notshown).

For purposes of illustration FIGS. 2A-2D only show a single space 250between the closed-curve thermal coupler 230 and the heating element240. It should be understood that, alternatively, a space 250 could bebetween the closed-curve waveguide 220 and the closed-curve thermalcoupler 230 but not between the closed-curve thermal coupler 230 and theheating element 240 or spaces 250 could be between the closed-curvethermal coupler 230 and both the closed-curve waveguide 220 and theheating element 240.

In the photonic structure embodiment 200A (see FIG. 2A), theclosed-curve waveguide 220 and the closed-curve thermal coupler 230 canbe continuous portions 203.1-203.2 of the same semiconductor layer 203on the insulator layer 202. That is, the closed-curve waveguide 220 canbe a first portion 203.1 of the semiconductor layer 203, which has afirst thickness (i.e., see the first height 225). The closed-curvethermal coupler 230 can be a second portion 203.2 of the semiconductorlayer 203 (also referred to herein as a recessed portion or slabportion), which is continuous with the first portion 203.1 but which hasbeen recessed (i.e., etched back) so as to have a second thickness(i.e., see the second height 235) that is less than the first portion(i.e., the second portion 203.2 is thinner than the first portion203.2). The heating element 240 can include a third portion 203.3 of thesemiconductor layer 203, which is physically separated from the secondportion 203.2 by a space 250 and can further include a metal silicidelayer 245 on the third portion 203.3.

The semiconductor layer 203 can be, for example, a silicon layer.Alternatively, the semiconductor layer could be some other type ofsemiconductor layer suitable for optical waveguide formation and havinga temperature-dependent refractive index including, but not limited to,a polysilicon layer, a germanium layer, or a silicon germanium layer.The metal silicide layer 245 can be, for example, a cobalt silicide(CoSi) layer, a nickel silicide (NiSi) layer, a tungsten silicide (WSi)layer, a titanium silicide (TiSi) layer, or any other suitable metalsilicide layer. Optionally, the metal silicide layer 145 could be dopedwith N-type or P-type dopants for reduced resistance.

In the photonic structure embodiment 200B (see FIG. 2B), theclosed-curve waveguide 220 and the closed-curve thermal coupler 230 canbe continuous portions 203.1-203.2 of the same semiconductor layer 203(e.g., the same silicon layer or, alternatively, the same polysilicon,germanium, or silicon germanium layer, as described above with respectto the photonic structure embodiment 200A). However, in this case, theheating element 240 can be some other metal or metal alloy resistiveelement 241 formed on the insulator layer 202 and physically separatedfrom the closed-curve thermal coupler 230 by a space 250. The resistiveelement 241 can be made, for example, of tungsten, aluminum, nickel,titanium, tantalum, cobalt, copper, or alloys thereof.

In the photonic structure embodiments 200C (see FIG. 2C) and 200D (seeFIG. 2D), the closed-curve waveguide 220 can be a discrete portion 203.1of a semiconductor layer 203 (e.g., a silicon layer or, alternatively, apolysilicon, germanium, or silicon germanium layer, as described abovewith respect to the photonic structure embodiment 200A). Theclosed-curve thermal coupler 230 can be some other non-metallicelectrically insulative thermally conductive feature formed on theinsulator layer 202 positioned laterally between the closed-curvewaveguide 220 and the heating element 240 and separated from one or bothby space(s) 250. Each space 250 can be sufficiently small such that theclosed-curve thermal coupler 230 is still thermally coupled to both theclosed-curve waveguide 220 and the heating element 240. In this case,the closed-curve thermal coupler 230 (i.e., the non-metallicelectrically insulative thermally conductive feature) can be, forexample, a thin layer of polysilicon, silicon, nitride, boron nitride,silicon carbide, diamond or some other non-metallic electricallyinsulative thermally conductive material on the insulator layer 202between the closed-curve waveguide 220 and the heating element 240. Theheating element 240 can include a discrete portion 203.3 of thesemiconductor layer 203 (e.g., positioned laterally immediately adjacentto the closed-curve thermal coupler 230 but separated therefrom by aspace 250) and can further include a metal silicide layer 245 on thediscrete patterned portion 203.3 of the semiconductor layer 203, asdescribed above with respect to the photonic structure embodiment 200A(e.g., see the photonic structure embodiment 200C of FIG. 2C).Alternatively, the heating element 240 can be some other metal or metalalloy resistive element 241 formed on the insulator layer 202 adjacentto the closed-curve thermal coupler 230 but separated therefrom by aspace 250, as described above with respect to the photonic structureembodiment 200B (e.g., see the photonic structure embodiment 200D ofFIG. 2D).

In each of the photonic structure embodiments 200A-200D, the opticaldevice 299 can be a ring resonator. The ring resonator can include atleast one closed-curve waveguide 220 and at least one bus waveguide 210adjacent to the closed-curve waveguide 220. In this case, the buswaveguide 210 can be a discrete portion 203.4 of the semiconductor layer203, which is adjacent to but spatially separated from the closed-curvewaveguide 220 (i.e., from the first portion 203.1 of the semiconductorlayer 203) and optically coupled thereto.

Referring to FIG. 3 and FIGS. 3A-3B, in the photonic structureembodiments 300A-300B, the heating element 340 can be at a differentdesign level than the closed-curve thermal coupler 330 and theclosed-curve waveguide 320.

More specifically, in the photonic structure embodiments 300A-300B, theheating element 340 can have a bottom surface, which is above andphysically separated from the top surface of the insulator layer 302, asshown. For example, the heating element 340 could be a metal or metalalloy resistive element 345 in a back end of the line (BEOL) metal level306. For example, the heating element 340 could be made of coper oraluminum or some other BEOL metal material. Alternatively, the heatingelement 340 could be below the insulator layer 302 (not shown). Forexample, the heating element 340 could be a metal or metal alloyresistive element embedded in the substrate 301) (not shown). In anycase, the heating element 340 can have essentially the same general loopor ring shape as the closed-curve thermal coupler and the closed-curvewaveguide and can be aligned with and sufficiently close to theclosed-curve thermal coupler 330 so as to be thermally coupled thereto.The heating element 340 can be a complete loop or ring shape with nodiscrete ends. Alternatively, the heating element 340 could have atleast one small segment removed so as to have an open-loop or ring shapewith discrete ends (not shown). Alternatively, heating element 340 couldbe segmented with the segments forming the loop or ring shape and witheach segment having discrete contacted ends (e.g., so that differentamounts of heat energy could be applied to different portions of theclosed-curve thermal coupler and thereby to different portions of theclosed-curve waveguide) (not shown). For purposes of illustration, theheating element 340 is shown as extending partially over theclosed-curve thermal coupler 330. However, alternatively, the heatingelement 340 could extend completely over the closed-curve thermalcoupler or could be completely offset from the closed-curve thermalcoupler.

In the photonic structure embodiment 300A (see FIG. 3A), theclosed-curve waveguide 320 and the closed-curve thermal coupler 330 eachinclude different continuous portions of the same semiconductor layer303 on the insulator layer 302. That is, the closed-curve waveguide 320can be a patterned first portion 303.1 of the semiconductor layer 303,which has a first thickness (i.e., see the first height 325). Theclosed-curve thermal coupler 330 can be a patterned second portion 303.2of the semiconductor layer 303 (also referred to herein as a recessedportion or slab portion), which is continuous with the first portion303.1 but which has been recessed (i.e., etched back) so as to have asecond thickness (i.e., see the second height 335) that is less than thefirst portion (i.e., the second portion 303.2 is thinner than the firstportion 303.2). The bus waveguide 310 can be a discrete patternedportion 303.4 of the semiconductor layer 303, which also has the firstthickness. The semiconductor layer 303 can be, for example, a siliconlayer. Alternatively, the semiconductor layer could be some other typeof semiconductor layer suitable for optical waveguide formation andhaving a temperature-dependent refractive index including, but notlimited to, a polysilicon layer, a germanium layer, or a silicongermanium layer.

In the photonic structure embodiments 300B (see FIG. 3B), theclosed-curve waveguide 320 and the bus waveguide 310 can be discretepatterned portions 303.1 and 303.4, respectively, of the samesemiconductor layer 303. The closed-curve thermal coupler 330 can besome other non-metallic electrically insulative thermally conductivefeature formed on the insulator layer 302 positioned laterally adjacentto and abutting the closed-curve waveguide 320. For example, theclosed-curve thermal coupler 330 could be a thin layer of polysilicon,silicon, nitride, boron nitride, silicon carbide, or diamond on theinsulator layer 302 positioned laterally adjacent to and abutting theclosed-curve waveguide 320.

In any case, each of the photonic structure embodiments 100A-100D,200A-200D, 300A-300B can further include one or more layers ofdielectric material 105, 205, 305 covering exposed surfaces of theinsulator layer 102, 202, 302 and any device components thereon. Forexample, the layer(s) of dielectric material can cover the opticaldevice 199, 299, 399 (including the closed-curve waveguide 120, 220, 320and, if applicable, the bus waveguide 110, 210, 310), the closed-curvethermal coupler 130, 230, 330 and, if applicable, the heating element(e.g., see heating element 140 and 240) and any spacers therebetween.That is, the dielectric material can fill any spaces between the buswaveguide and the closed-curve waveguide, between the closed-curvewaveguide and the closed-curve thermal coupler, and/or between theclosed-curve thermal coupler and the heating element. This ddielectricmaterial 105, 205, 305 and, particularly, the dielectric material thatis immediately adjacent to the optical waveguide(s) (i.e., immediatelyadjacent to the closed-curve waveguide 120, 220, 320 and the buswaveguide 110, 210, 310, if applicable) can be any dielectric materialthat is suitable for use as cladding material for those opticalwaveguide(s). For example, if the optical waveguide(s) are siliconwaveguide(s), which as discussed above can has a temperature andwavelength-dependent refractive index that is typically above 3.2, thenthe layer of dielectric material 105, 205, 305 that is immediatelyadjacent to those optical waveguide(s) could be silicon dioxide (e.g.,with a refractive index of less than 1.6), silicon nitride (e.g., with arefractive index of less than 2.1), or any other suitable dielectricmaterial that has a refractive index that is less than the lowestrefractive index of silicon.

In each of the photonic structure embodiments 100A-100D, 200A-200D,300A-300B, described above and illustrated in the drawings, the opticaldevice 199, 299, 399 is shown as being a simple ring resonator thatincludes a single bus waveguide 110, 210, 310 and a single closed-curvewaveguide 120, 220, 320. However, it should be understood that thefigures are not intended to be limiting. Alternatively, the opticaldevice 199, 299, 399 could be a ring resonator with some other morecomplex configuration (e.g., multiple closed-curve waveguides opticallycoupled to the same bus waveguide, a single closed-curve waveguidebetween and optically coupled to a pair of parallel bus waveguides,multiple closed-curve waveguides between and optically coupled to a pairof parallel bus waveguides, etc.). In such complex ring resonatorconfigurations, each closed-curve waveguide can be indirectly thermallycoupled to a corresponding heating element via a correspondingclosed-curve thermal coupler, as described above.

Finally, as mentioned above, in each of the photonic structureembodiments 100A-100D, 200A-200D, 300A-300B, the outer curved sidewall121, 221, 321 of the closed-curve waveguide 120, 220, 320 extendsessentially vertically the full first height 125, 225, 325 of theclosed-curve waveguide from the top surface of the insulator layer 102,202, 302 to the top surface of the waveguide itself. As a result, modeconfinement is improved, and signal loss is minimized. Moreparticularly, conventional waveguides are typically either strip or ribwaveguides. A strip waveguide refers to a waveguide that is formed, forexample, by forming a mask with the desired shape on a waveguide corematerial layer and then performing an anisotropic etch process to etchcompletely through the waveguide core material layer so that allsidewalls of the resulting waveguide extend vertically the full heightof the waveguide. A rib waveguide refers to a waveguide that is formed,for example, by forming a mask with the desired shape on a waveguidecore material layer and then performing an anisotropic etch process toetch only partially through a waveguide core material layer so thatrecessed portions (also referred to as slab portions) of the waveguidecore material extend laterally away from a lower portion of theresulting waveguide and so that the sidewalls of the resulting waveguidedo not extend the full height of the waveguide but instead only extendfrom vertically from the recessed portion of the waveguide corematerial. Thus, a rib waveguide essentially has an inverted T-shape. Therecessed portions of the waveguide core material in a rib waveguideallow for light signal leakage. Furthermore, in closed-curve ribwaveguides, the potential for light signal leakage is greater along theouter curved sidewall. Thus, by eliminating recessed portions ofwaveguide core material (e.g., silicon or, alternatively, polysilicon,germanium or silicon germanium) at least along the outer curved sidewall121, 221, 321 of the closed-curve waveguide 120, 220, 320 in each of thephotonic structure embodiments 100A-100D, 200A-200D, 300A-300B, modeconfinement is improved and signal loss is minimized at least to someextent even in embodiments where the closed-curve thermal coupler 130,230, 330 is a continuous, but recessed, portion 103.2, 203.2, 303.2 ofsame semiconductor layer 103, 203, 303 (e.g., the same silicon layer or,alternatively, the same polysilicon, germanium or silicon germaniumlayer) used to form the closed-curve waveguide 120, 220, 320 (e.g., asin the photonic structure embodiments 100A-100B of FIGS. 1A-1B,200A-200B of FIGS. 2A-2B and 300A of FIG. 3B).

Referring to the flow diagram of FIG. 4 , also disclosed herein areembodiments of a method of forming the above-described photonicstructure embodiments (e.g., see the photonic structure embodiments100A-100D shown in the layout diagram of FIG. 1 and further illustratedin the alternative cross-section diagrams of FIGS. 1A-1D, respectively;see the photonic structure embodiments 200A-200D shown in the layoutdiagram of FIG. 2 and further illustrated the alternative cross-sectiondiagrams of FIGS. 2A-2D, respectively; and see the photonic structureembodiments 300A-300B shown in the layout diagram of FIG. 3 and furtherillustrated in the alternative cross-section diagrams of FIGS. 3A-3B).

Generally, the method embodiments can include forming an optical device199, 299, 399 on an insulator layer 102, 202, 302 (see process step402). The process of forming the optical device 199, 299, 399 caninclude at least forming a closed-curve waveguide 120, 220, 320. Forexample, in some embodiments, the process of forming the optical device199, 299, 399 can include forming a ring resonator, which includes atleast one bus waveguide 110, 210, 310 and at least one closed-curvewaveguide 120, 220, 320 that is positioned laterally adjacent andoptically coupled to the bus waveguide 110, 210, 310. As mentionedabove, for purposes of this disclosure, a bus waveguide refers to awaveguide with discrete ends including an input end and an output end,whereas a closed-curve waveguide refers to a waveguide with a completeloop or ring shape with no discrete ends. In any case, the closed-curvewaveguide 120, 220, 320 of the optical device 199, 299, 399 can beformed at process step 402 so as to have a first height 125, 225, 325(e.g., as measured from a top surface of an insulator layer to a topsurface of the closed-curve waveguide), an outer curved sidewall 121,221, 321 that extends essentially vertically the full first height ofthe closed-curve waveguide (e.g., in order to improve mode confinementand minimize signal loss), and an inner curved sidewall 122, 222, 322opposite the outer curved sidewall. Furthermore, the dimensions of theclosed-curve waveguide 120, 220, 320 can be customized to achievedesired results. For example, the height and width of the closed-curvedwaveguide can be customized to facilitate propagation of light signalshaving wavelengths within a given wavelength range. Furthermore, thecircumference of the closed-curve waveguide can be customized to achievea specific resonant wavelength and to set how often resonance occurs. Asmentioned above, the resonant wavelength of a closed-curve waveguiderefers to the wavelength of light signals that will make repeatedroundtrips through the closed-curve waveguide, building up intensity.Techniques for customizing the dimensions of waveguides are well knownin the art and, thus, the details have been omitted from thisspecification in order to allow the reader to focus on the salientaspects of the disclosed embodiments.

The method can further include forming a closed-curve thermal coupler130, 230, 330 (see process step 404). This closed-curve thermal coupler130, 230, 330 can be formed so as to be smaller than the closed-curvewaveguide 120, 220, 320 with essentially the same complete loop or ringshape and further so as to be laterally surrounded by and thermallycoupled to the closed-curve waveguide 120, 220, 320. Thermal couplingbetween the closed-curve waveguide 120, 220, 320 and the closed-curvethermal coupler 130, 230, 330 can be achieved by forming theclosed-curve thermal coupler 130, 230, 330 such that it abuts or issufficiently close to the closed-curve waveguide 120, 220, 320 to ensurethat heat energy from the closed-curve thermal coupler 130, 230, 330 canpass into the closed-curve waveguide 120, 220, 320. Furthermore, thisclosed-curve thermal coupler 130, 230, 330 can be formed so as to have asecond height that is less than the first height of the closed-curvewaveguide 120, 220, 320. For purposes of this disclosure, a “thermalcoupler” refers to a non-contacted component made of any suitablenon-metallic electrically insulative thermally conductive materialthrough which heat energy can be transmitted without also transmittingelectric current. Exemplary thermal coupler materials are discussed ingreater detail below with respect to specific method embodiments.

The method can further include forming a heating element 140, 240, 340to facilitate thermal tuning of the closed-curve waveguide 120, 220, 320(see process step 406). For purposes of this disclosure, a “heatingelement” refers to a resistor made of any suitable conductive materialthrough which electric current flows in response to a voltagedifferential at the contacts and is converted into heat energy. Thoseskilled in the art will recognize that the direction and amount ofcurrent flow will depend upon the voltage differential. Furthermore, theamount of heat generated per unit length will depend upon the materialused and the current density (which is a function of the cross-sectionalarea of the heating element). In this case, instead of forming theheating element 140, 240, 340 such that it is directly thermally coupledto the closed-curve waveguide for thermal tuning, the heating element140, 240, 340 can be formed such that it is thermally coupled to theclosed-curve thermal coupler 130, 230, 330 and thereby indirectlythermally coupled to the closed-curve waveguide 120, 220, 320. Forexample, the heating element can be formed at the same design level or adifferent design level as the closed-curve thermal coupler 130, 230,330. Thermal coupling between the heating element 140, 240, 240 and theclosed-curve thermal coupler 130, 230, 330 can be achieved by formingthe heating element 140, 240, 340 such that it abuts or is sufficientlyclose to the closed-curve thermal coupler 130, 230, 330 to ensure thatheat energy from the heating element 140, 240, 340 can pass into theclosed-curve thermal coupler 130, 230, 330. Exemplary heating elementmaterials are discussed in greater detail below with regard to thespecific embodiments.

The various method embodiments can vary with regard to the specificprocessing techniques and/or materials used during process steps 402-408to form the photonic structure embodiments 100A-100D, 200A-200D,300A-300B and more particularly to form the different components (e.g.,for the waveguide(s), the thermal coupler, and the heating element)thereof.

For example, referring to FIG. 1 and also FIGS. 1A-1D, in someembodiments of the method, process steps 402-408 can be performed sothat the heating element 140, the closed-curve thermal coupler 130, theclosed-curve waveguide 120 and, if applicable, the bus waveguide 110 areall at the same design level and, particularly, immediately adjacent tothe top surface of the insulator layer 102 and further so that theclosed-curve thermal coupler 130 is positioned laterally between andimmediately adjacent to both the closed-curve waveguide 120 and theheating element 140.

Exemplary process steps for forming the photonic structure embodiment100A of FIG. 1A are illustrated in FIGS. 5A-5C. For example, a mask 501can be formed on the semiconductor layer 103 (e.g., using conventionallithographic patterning and etch techniques or any other suitable maskformation techniques) and an anisotropic etch process can be performedin order to define, in the semiconductor layer 103, the initial shapesfor a bus waveguide 110 (see portion 103.4), for a closed-curvewaveguide 120 (see portion 103.1) and for a heating element 140 (seeportion 103.3) each having a first height 125 and doing so withoutetching completely through the semiconductor layer 103, thereby leavinga recessed portion 103.5 of the semiconductor layer 103 (also referredas a slab portion) with a second height 135 that is less than the firstheight 125 covering the insulator layer 102 between the thickerpatterned portions 103.1, 103.3 and 103.4 (see FIG. 5A). The mask 501can be removed. Then, another mask 502 can be formed on the partiallycompleted structure (e.g., using conventional lithographic patterningand etch techniques or any other suitable mask formation techniques) andan anisotropic etch process can then be performed to completely removeexposed sections of the recessed portion of the semiconductor layer(e.g., those sections immediately adjacent to opposing sides of the buswaveguide and immediately adjacent to the outer curved sidewall of theclosed-curve waveguide), leaving intact a recessed portion 103.2 of thesemiconductor layer 103 for the closed-curve thermal coupler 130extending laterally between and immediately adjacent to the thickpatterned portions 103.1 and 103.3 of the semiconductor layer (i.e.,extending laterally between and immediately adjacent to the closed-curvewaveguide 120 and the heating element 140) (see FIG. 5B). The mask 502can be removed. Then another mask 503 can be formed on the partiallycompleted structure (e.g., using conventional lithographic patterningand etch techniques or any other suitable mask formation techniques) anda metal silicide process can be performed in order to form a metalsilicide layer 145 for the heating element 140 on exposed surfaces ofthe thick patterned portion 103.3 of the semiconductor layer. The metalsilicide layer 145 could be, for example, a cobalt silicide (CoSi)layer, a nickel silicide (NiSi) layer, a tungsten silicide (WSi) layer,a titanium silicide (TiSi) layer, or any other suitable metal silicidelayer. Optionally, the metal silicide layer 145 could be doped withN-type or P-type dopants for reduced resistance. Techniques for formingmetal silicide layers are well known in the art and, thus, omitted fromthis specification in order to allow the reader to focus on the salientaspects of the disclosed embodiments.

As a result of the process steps shown in FIGS. 5A-5C and describedabove, the closed-curve waveguide 120, the closed-curve thermal coupler130, and the heating element 140 include continuous portions 103.1-103.3of the same semiconductor layer 103 (e.g., the same silicon layer or,alternatively, the same polysilicon, germanium, or silicon germaniumlayer) on the insulator layer 102. That is, the closed-curve waveguide120 is made of a first portion 103.1 of the semiconductor layer 103,which has a first thickness (i.e., see the first height 125). Theclosed-curve thermal coupler 130 is made of a second portion 103.2 ofthe semiconductor layer 103 (also referred to herein as a recessedportion or slab portion), which is continuous with the first portion103.1 but which has been recessed (i.e., etched back) so as to have asecond thickness (i.e., see the second height 135) that is less than thefirst portion (i.e., the second portion 103.2 is thinner than the firstportion 103.2). The heating element 140 includes both a third portion103.3 of the semiconductor layer 103, which is continuous with thesecond portion 103.2, and a metal silicide layer 145 on the thirdportion 103.3.

Exemplary process steps for forming the photonic structure embodiment100B of FIG. 1B are illustrated in FIGS. 6A-6C. For example, a mask 601can be formed on the semiconductor layer 103 (e.g., using conventionallithographic patterning and etch techniques or any other suitable maskformation techniques) and an anisotropic etch process can be performedin order to define, in the semiconductor layer 103, the initial shapesfor a bus waveguide 110 (see portion 103.4) and for a closed-curvewaveguide 120 (see portion 103.1) each having a first height 125 anddoing so without etching completely through the semiconductor layer 103,thereby leaving a recessed portion 103.5 of the semiconductor layer 103(also referred as a slab portion) with a second height 135 that is lessthan the first height 125 covering the insulator layer 102 between thethicker patterned portions 103.1 and 103.4 (see FIG. 6A). The mask 601can be removed. Then, another mask 602 can be formed on the partiallycompleted structure (e.g., using conventional lithographic patterningand etch techniques or any other suitable mask formation techniques) andan anisotropic etch process can then be performed to completely removespecific sections of the recessed portion of the semiconductor layer(e.g., immediately adjacent to opposing sides of the bus waveguide,immediately adjacent to the outer curved sidewall of the closed-curvewaveguide and spatially separated from the inner curved sidewall of theclosed-curve waveguide), leaving intact a recessed portion 103.2 of thesemiconductor layer 103 for the closed-curve thermal coupler 130positioned laterally immediately to the thick patterned portion 103.1 ofthe semiconductor layer (i.e., positioned laterally immediately adjacentto the closed-curve waveguide 120) (see FIG. 6B). The mask 602 can beremoved. Then another mask 603 can be formed on the partially completedstructure (e.g., using conventional lithographic patterning and etchtechniques or any other suitable mask formation techniques) and a metalor metal alloy resistive element 141 for the heating element 140 can beformed (e.g., deposited into a patterned opening in the mask) above andimmediately adjacent to the insulator layer 102 and further positionedlaterally immediately adjacent to the recessed portion 103.2 of thesemiconductor layer (i.e., so as to abut the closed-curve thermalcoupler 130). This resistive element 141 can be made, for example, oftungsten, aluminum, nickel, titanium, tantalum, cobalt, copper, oralloys thereof. As a result of the process steps shown in FIGS. 6A-6Cand described above, the closed-curve waveguide 120 and the closed-curvethermal coupler 130 are continuous portions 103.1-103.2 of the samesemiconductor layer 103, but the heating element 140 is a discrete metalor metal alloy feature.

Exemplary process steps for forming the photonic structure embodiment100C of FIG. 1C are illustrated in FIGS. 7A-7C. For example, a mask 701can be formed on the semiconductor layer 103 (e.g., using conventionallithographic patterning and etch techniques or any other suitable maskformation techniques) and an anisotropic etch process can be performedin order to define, in the semiconductor layer 103, discrete shapes fora bus waveguide 110 (see portion 103.4), for a closed-curve waveguide120 (see portion 103.1) and for a heating element 140 (see portion103.3) each having a first height 125 and doing so in a manner thatcompletely etches through the semiconductor layer 103, thereby exposingportions of the insulator layer 102 between the patterned portions103.1, 103.3 and 103.4 (see FIG. 7A). The mask 701 can be removed. Then,another mask 702 can be formed on the partially completed structure(e.g., using conventional lithographic patterning and etch techniques orany other suitable mask formation techniques) and a non-metallicelectrically insulative thermally conductive material can be depositedinto an opening in the mask 702 between the patterned portions 103.1 and103.2 of the semiconductor layer and recessed so as to have a secondheight 135 that is less than the first height, thereby forming anon-metallic electrically insulative thermally conductive feature 104for a closed-curve thermal coupler 130 (see FIG. 7B). Exemplarynon-metallic electrically insulative thermally conductive materials thatcould be used include, but are not limited to, polysilicon, boronnitride, silicon carbide, or diamond. The mask 702 can be removed. Thenyet another mask 703 can be formed on the partially completed structure(e.g., using conventional lithographic patterning and etch techniques orany other suitable mask formation techniques) and a metal silicideprocess can be performed in order to form a metal silicide layer 145 forthe heating element 140 on exposed surfaces of the thick patternedportion 103.3 of the semiconductor layer. As discussed above, the metalsilicide layer 145 could be, for example, any of a cobalt silicide(CoSi) layer, a nickel silicide (NiSi) layer, a tungsten silicide (WSi)layer, a titanium silicide (TiSi) layer, or any other suitable metalsilicide layer. Optionally, the metal silicide layer 145 could be dopedwith N-type or P-type dopants for reduced resistance.

Exemplary process steps for forming the photonic structure embodiment100D of FIG. 1D are illustrated in FIGS. 8A-8C. For example, a mask 801can be formed on the semiconductor layer 103 (e.g., using conventionallithographic patterning and etch techniques or any other suitable maskformation techniques) and an anisotropic etch process can be performedin order to define, in the semiconductor layer 103, discrete shapes fora bus waveguide 110 (see portion 103.4) and for a closed-curve waveguide120 (see portion 103.1) each having a first height 125 and doing so in amanner that completely etches through the semiconductor layer 103,thereby exposing portions of the insulator layer 102 between andadjacent to the patterned portions 103.1 and 103.4 (see FIG. 7B). Themask 801 can be removed. Then, another mask 802 can be formed on thepartially completed structure (e.g., using conventional lithographicpatterning and etch techniques or any other suitable mask formationtechniques) and a non-metallic electrically insulative thermallyconductive material can be deposited into an opening in the mask 802adjacent to an inner curved sidewall of the patterned portion 103.1 andrecessed so as to have a second height 135 that is less than the firstheight, thereby forming a non-metallic electrically insulative thermallyconductive feature 104 for a closed-curve thermal coupler 130 (see FIG.8B). As discussed above, non-metallic electrically insulative thermallyconductive materials that could be used include, but are not limited to,polysilicon, silicon, nitride, boron nitride, silicon carbide, ordiamond. The mask 802 can be removed. Then yet another mask 803 can beformed on the partially completed structure (e.g., using conventionallithographic patterning and etch techniques or any other suitable maskformation techniques) and a metal or metal alloy resistive element 141for the heating element 140 can be formed (e.g., deposited into apatterned opening in the mask) above and immediately adjacent to theinsulator layer 102 and further positioned laterally immediatelyadjacent to the non-metallic electrically insulative thermallyconductive feature 104 (i.e., so as to abut the closed-curve thermalcoupler 130). As discussed above, such a resistive element 141 can bemade, for example, of tungsten, aluminum, nickel, titanium, tantalum,cobalt, copper, or alloys thereof.

The process steps described above and illustrated in FIGS. 5A-5C, 6A-6C,7A-7C or 8A-8C are provided for illustration purposes and are notintended to be limiting. The order of the process steps could vary anyand/or alternative process steps could be performed to form the desiredcomponents. In any case, the process steps described above andillustrated in FIGS. 5A-5C, 6A-6C, 7A-7C or 8A-8C can be followed bydeposition of one or more layers of dielectric material 105 to coverexposed surfaces of the insulator layer 102 and any device componentsthereon. For example, the layer(s) of dielectric material can bedeposited so as to cover the optical device 199 (including theclosed-curve waveguide 120 and, if applicable, the bus waveguide 110),the closed-curve thermal coupler 130 and the heating element 140. Thisdielectric material 105 and, particularly, any dielectric materialdeposited directly onto the optical waveguide(s) can be any dielectricmaterial that is suitable for use as cladding material for those opticalwaveguide(s). For example, if the optical waveguide(s) are siliconwaveguide(s), which as discussed above can has a temperature andwavelength-dependent refractive index that is typically above 3.2, thenthe layer of dielectric material 105 that is immediately adjacent tothose optical waveguide(s) could be silicon dioxide (e.g., with arefractive index of less than 1.6), silicon nitride (e.g., with arefractive index of less than 2.1), or any other suitable dielectricmaterial that has a refractive index that is less than the lowestrefractive index of silicon.

It should be understood that similar processes could be performed toform the photonic structure embodiments 200A-200D of FIGS. 2 and 2A-2Dexcept that the various patterning processes can be performed to ensurethat the closed-curve thermal coupler 230 is physically separated fromthe closed-curve waveguide 220 and/or from the heating element 240 byspace(s) 250. Additionally, it should be understood that similarprocesses could be performed in order to form the photonic structureembodiments 300A-300B of FIGS. 3 and 3A-3B except that formation of theheating element 340 will be performed only during BEOL processing afterdeposition of the dielectric material 305 over the optical device 399(including the closed-curve waveguide 320 and, if applicable, the buswaveguide 310) and the closed-curve thermal coupler 330 in a lowerdesign level.

Referring again to FIG. 4 , the method can further include using theheating element 140, 240, 340 to thermally tune the closed-curvewaveguide 120, 220, 320 via the closed-curve thermal coupler 130, 230,330 (e.g., using the heating element to generate and output heat energy,which passes into and through the closed-curve thermal coupler and whichfurther passes into the closed-curve waveguide) in order to minimize anytemperature-dependent resonance shift (TDRS) (see process step 408).That is, during thermal tuning, heat energy can be generated and outputby the heating element 140, 240, 340 and, due to thermal coupling, canbe passed into the closed-curve thermal coupler 130, 230, 330, cantravel through the closed-curve thermal coupler 130, 230, 330, and canpass from the closed-curve thermal coupler 130, 230, 330 into the lowerportion of the closed-curve waveguide 120, 220, 320. The amount of heatenergy can be predetermined to ensure that the temperature of theclosed-curve waveguide 120, 220, 320 is maintained at a specifictemperature or within a specific temperature range so as to ensure thatlight signals of the specific resonant wavelength will build upintensity as they make repeated roundtrips through the closed-curvewaveguide 120, 220, 320.

It should be noted that the method embodiments can further include,prior to forming the various components (i.e., the closed-curvewaveguide, the closed-curve thermal coupler, and the heating element),predetermining the dimensions of the various components as well as thematerials of the components and any spacing between the components toensure that the desired resonant wavelength can be achieve throughthermal tuning of the closed-curve waveguide by the heating elementthrough the closed-curve thermal coupler can be achieved.

The method as described above is used in the fabrication of integratedcircuit chips. The resulting integrated circuit chips can be distributedby the fabricator in raw wafer form (that is, as a single wafer that hasmultiple unpackaged chips), as a bare die, or in a packaged form. In thelatter case the chip is mounted in a single chip package (such as aplastic carrier, with leads that are affixed to a motherboard or otherhigher-level carrier) or in a multichip package (such as a ceramiccarrier that has either or both surface interconnections and buriedinterconnections). In any case the chip is then integrated with otherchips, discrete circuit elements, and/or other signal processing devicesas part of either (a) an intermediate product, such as a motherboard, or(b) an end product. The end product can be any product that includesintegrated circuit chips, ranging from toys and other low-endapplications to advanced computer products having a display, a keyboardor other input device, and a central processor.

In some of the photonic structure and method embodiments describedabove, some components (e.g., the closed curved-waveguide, the closed-curve thermal coupler, and a part of the heating element, as in FIG. 1A;or the closed-curve waveguide and closed-curve thermal coupler, as inFIGS. 1B, 2A, 2B or 3A) are referred to as being continuous portions ofthe same semiconductor layer. It should be understood that any two“continuous portions of the same semiconductor layer” that make up anytwo components, respectively, of the photonic structure will be portionsof a semiconductor layer that are immediately adjacent to each other butotherwise processed so as to be distinguishable, as described above. Forexample, the semiconductor layer can be formed on (e.g., deposited onto)the insulator layer and subsequently processed in such a way that thetwo portions at issue are not physically separated but have differentdimensions and, particularly, different heights and shapes so as to bedistinguishable, as described above. Thus, for example, in theembodiments shown in FIGS. 1A, 1B, 2A, 2B, and 3A, the closed-curvewaveguide 120, 220, 320 and the closed-curve thermal coupler 130, 230,330 are continuous portions of the same semiconductor layer with theclosed-curve waveguide 120, 220, 320 being a first portion 103.1, 203.1,303.1 of the semiconductor layer and the closed-curve thermal couplerbeing a second portion 103.2, 203.2, 303.2 that is thinner than thefirst portion 103.1, 203.1, 303.1 and that extends laterally fromimmediately adjacent to the first portion 103.1, 203.1, 303.1.

Additionally, in the photonic structure and method embodiments describedabove, some components (e.g., the bus waveguide in FIGS. 1A, 1B, 2B, and3A; the bus waveguide, the closed curved-waveguide, and a part of theheating element, as in FIG. 1C and 2C; and the bus waveguide and theclosed curved-waveguide, as in FIGS. 1D, 2D and 3B) are described asbeing discrete portions of the same semiconductor layer. It should beunderstood that a “discrete portion” of a semiconductor layer that makesup a component of the photonic structure is a patterned portion of thesemiconductor layer that is physically separated from all other portionsso as to be distinguishable, as described above. For example, thesemiconductor layer can be formed on (e.g., deposited onto) theinsulator layer and subsequently processed in such a way that theportion at issue is physically separated from all other portions and hasthe desired dimensions, as described above.

It should be understood that the terminology used herein is for thepurpose of describing the disclosed structures and methods and is notintended to be limiting. For example, as used herein, the singular forms“a”, “an” and “the” are intended to include the plural forms as well,unless the context clearly indicates otherwise. Additionally, as usedherein, the terms “comprises” “comprising”, “includes” and/or“including” specify the presence of stated features, integers, steps,operations, elements, and/or components, but do not preclude thepresence or addition of one or more other features, integers, steps,operations, elements, components, and/or groups thereof. Furthermore, asused herein, terms such as “right”, “left”, “vertical”, “horizontal”,“top”, “bottom”, “upper”, “lower”, “under”, “below”, “underlying”,“over”, “overlying”, “parallel”, “perpendicular”, etc., are intended todescribe relative locations as they are oriented and illustrated in thedrawings (unless otherwise indicated) and terms such as “touching”, “indirect contact”, “abutting”, “directly adjacent to”, “immediatelyadjacent to”, etc., are intended to indicate that at least one elementphysically contacts another element (without other elements separatingthe described elements). The term “laterally” is used herein to describethe relative locations of elements and, more particularly, to indicatethat an element is positioned to the side of another element as opposedto above or below the other element, as those elements are oriented andillustrated in the drawings. For example, an element that is positionedlaterally adjacent to another element will be beside the other element,an element that is positioned laterally immediately adjacent to anotherelement will be directly beside the other element, and an element thatlaterally surrounds another element will be adjacent to and border theouter sidewalls of the other element. The corresponding structures,materials, acts, and equivalents of all means or step plus functionelements in the claims below are intended to include any structure,material, or act for performing the function in combination with otherclaimed elements as specifically claimed.

The descriptions of the various embodiments of the present inventionhave been presented for purposes of illustration but are not intended tobe exhaustive or limited to the embodiments disclosed. Manymodifications and variations will be apparent to those of ordinary skillin the art without departing from the scope and spirit of the describedembodiments. The terminology used herein was chosen to best explain theprinciples of the embodiments, the practical application or technicalimprovement over technologies found in the marketplace, or to enableothers of ordinary skill in the art to understand the embodimentsdisclosed herein.

What is claimed is:
 1. A structure comprising: a closed-curve waveguidehaving a first height; a closed-curve thermal coupler laterallysurrounded by the closed-curve waveguide, wherein the closed-curvethermal coupler has a second height that is less than the first height;and a heating element adjacent to the closed-curve thermal coupler,wherein the heating element is separated from the closed-curve thermalcoupler by dielectric material, and wherein the closed-curve thermalcoupler is adapted to pass heat energy from the heating element to theclosed-curved waveguide.
 2. The structure of claim 1, wherein a bottomof the heating element is above a top of the closed-curve thermalcoupler.
 3. The structure of claim 1, wherein the heating elementextends laterally over a portion of the closed-curve thermal coupler. 4.The structure of claim 1, further comprising a dielectric layer on theclosed-curve waveguide and on the closed-curve thermal coupler, whereinthe dielectric layer has a planar top surface and wherein the heatingelement is on the planar top surface.
 5. The structure of claim 4,wherein the heating element is a metal heating element.
 6. The structureof claim 1, wherein the closed-curve waveguide, the closed-curve thermalcoupler and the heating element are ring-shaped with the closed-curvewaveguide being larger than the closed-curve thermal coupler and theclosed-curve thermal coupler being larger than the heating element, andwherein an outer edge portion of the heating element overlays theclosed-curve thermal coupler.
 7. The structure of claim 1, furthercomprising an insulator layer, wherein the closed-curve waveguide andthe closed-curve thermal coupler are above and immediately adjacent to atop surface of the insulator layer, and wherein the closed-curvewaveguide has an outer curved sidewall that extends essentiallyvertically from the top surface of the insulator layer to the firstheight.
 8. The structure of claim 7, wherein the closed-curve thermalcoupler abuts the closed-curve waveguide.
 9. The structure of claim 7,further comprising a semiconductor layer on the insulator layer, whereinthe closed-curve waveguide comprises a first portion of a semiconductorlayer and the closed-curve thermal coupler comprises a second portion ofthe semiconductor layer that extends laterally from the first portionand is thinner than the first portion.
 10. The structure of claim 9,wherein the semiconductor layer comprises any of a silicon layer, apolysilicon layer, a germanium layer, and a silicon germanium layer. 11.A structure comprising: a ring resonator comprising: a bus waveguidehaving a first height; and a closed-curve waveguide adjacent to the buswaveguide and having the first height; a closed-curve thermal couplerlaterally surrounded by the closed-curve waveguide, wherein theclosed-curve thermal coupler has a second height that is less than thefirst height; and a heating element adjacent to the closed-curve thermalcoupler, wherein the heating element is separated from the closed-curvethermal coupler by dielectric material, and wherein the closed-curvethermal coupler is adapted to pass heat energy from the heating elementto the closed-curved waveguide.
 12. The structure of claim 11, furthercomprising a dielectric layer on the closed-curve waveguide and on theclosed-curve thermal coupler, wherein the dielectric layer has a planartop surface and wherein the heating element is on the planar topsurface.
 13. The structure of claim 12, wherein the heating element is ametal heating element.
 14. The structure of claim 11, wherein theclosed-curve waveguide, the closed-curve thermal coupler and the heatingelement are ring-shaped with the closed-curve waveguide being largerthan the closed-curve thermal coupler and the closed-curve thermalcoupler being larger than the heating element, and wherein an outer edgeportion of the heating element overlays the closed-curve thermalcoupler.
 15. The structure of claim 11, further comprising an insulatorlayer, wherein the closed-curve waveguide and the closed-curve thermalcoupler are above and immediately adjacent to a top surface of theinsulator layer, and wherein the closed-curve waveguide has an outercurved sidewall that extends essentially vertically from the top surfaceof the insulator layer to the first height.
 16. The structure of claim15, further comprising a semiconductor layer on the insulator layer,wherein the closed-curve waveguide comprises a first portion of asemiconductor layer and the closed-curve thermal coupler comprises asecond portion of the semiconductor layer that extends laterally fromthe first portion and is thinner than the first portion.
 17. A methodcomprising: forming a closed-curve waveguide having a first height;forming a closed-curve thermal coupler laterally surrounded by theclosed-curve waveguide and having a second height that is less than thefirst height; and forming a heating element adjacent to the closed-curvethermal coupler and further separated from the closed-curve thermalcoupler by dielectric material, wherein the closed-curve waveguide, theclosed-curve thermal coupler and the heating element are formed so thatheat energy generated by the heating element passes through theclosed-curve thermal coupler to the closed-curve thermal waveguide. 18.The method of claim 17, wherein the closed-curve waveguide, theclosed-curve thermal coupler and the heating element are formed so as tobe ring-shaped with the closed-curve waveguide being larger than theclosed-curve thermal coupler and the closed-curve thermal coupler beinglarger than the heating element, and wherein the heating element isfurther formed so that an outer edge portion of the heating elementoverlays the closed-curve thermal coupler.
 19. The method of claim 17,wherein the closed-curve waveguide and the closed-curve thermal couplerare formed above and immediately adjacent to a top surface of aninsulator layer, wherein the closed-curve waveguide is formed to have anouter curved sidewall that extends essentially vertically from the topsurface of the insulator layer to the first height, and wherein theheating element is formed so that a bottom of the heating element isabove a top of the closed-curve thermal coupler.
 20. The method of claim17, further comprising patterning a semiconductor layer on an insulatorlayer so the closed-curve waveguide comprises a first portion of thesemiconductor layer and the closed-curve thermal coupler comprises asecond portion of the semiconductor layer that extends laterally fromthe first portion and is thinner than the first portion.